Pluripotent stem cells are cells having the ability to differentiate into every cell present in the living body. Embryonic stem cells (ES cells) are a typical example thereof. Human ES cells are expected to be applied to regenerative medicine through the use of this property. The transplantation of differentiated ES cells, however, causes undesired rejection.
In recent years, the group of Yamanaka et al. has reported the development of so-called iPS cells (induced pluripotent stem cells), which are cells having pluripotency or a proliferative potential close to that of ES cells, by inducing dedifferentiation through the expression of 4 factors (Oct3/4, Sox2, Klf4, and c-myc) using mouse somatic cells (non-patent document 1), and then reported that the iPS cells can also be prepared from differentiated human cells (non-patent document 2). Such human iPS cells can be prepared using cells derived from patients to be treated and are therefore expected as tools for preparing artificial organs free from rejection. Nonetheless, the analysis of the in vivo behaviors of the iPS cells has suggested the possibility that the iPS cells are not necessarily cells having the same properties as those of ES cells. For example, as a result of preparing chimeric mice using iPS cells, tumor formation was observed in approximately 20% individuals. This is a significantly higher numeric value than that obtained in a similar experiment using ES cells.
To solve this problem of the high risk of tumor formation, it has been reported that: iPS cells can be prepared using only 3 factors (Oct3/4 gene, Sox2 gene, and Klf4 gene) without the use of c-myc known as an oncogene; and the risk of tumor formation can be reduced by the preparation of chimeric mice using the iPS cells (non-patent documents 3 and 4). However, the risk of tumor formation as close to zero as possible is required for the clinical application of pluripotent stem cells such as human iPS cells. Therefore, the risk of tumorigenic transformation is still viewed as a problem for the clinical application of iPS cells.
Meanwhile, studies are also ongoing to directly isolate pluripotent stem cells from living tissues. It has been reported that: a stress such as trypsin or hypoxic treatment can be applied to human bone marrow mesenchymal cells to thereby select stress-resistant pluripotent stem cells; and pluripotent stem cells can be selected with the expression of a pluripotent stem cell surface antigen SSEA-3 as an index and further isolated by repeated suspension culture (patent document 1 and non-patent document 5). These methods, however, require the operation of applying a stress to cells or selecting pluripotent stem cells with the expression of SSEA-3 as an index and are therefore susceptible to improvement in terms of time-effectiveness or cost-effectiveness.